A thermal recording medium, using a thermal coloring reaction of a substantially colorless or light-colored basic chromogenic dye precursor, a developer for causing it to color, a sensitizer, etc., is, as a simple and low priced recording system, widely used in the fields of a thermal facsimile, a thermal printer, a thermal label, and the like. In the above thermal recording media, essentially, a sensitizer is incorporated into a recording layer for improving its thermal colorability. However, thermal colorability and background fogging are often contradictory to each other. In a medium, the more improved in the thermal colorability, the more background fogging is liable to occur. On the other hand, when the more attempt is made to suppress the background fogging, the more deteriorated its colorability is. There is, therefore, strongly demanded to provide a sensitizer that improves both of these two properties in a thermal recording medium.
For complying with the above fundamental demand, a thermal recording medium, using a 1,2-bis(3-methylphenoxy)ethane compound as a sensitizer, has an established reputation as being excellently balanced between the above thermal colorability and background fogging as is described in Japanese Patent No. 1804948. And the above compound is, and has been, widely produced, sold and used for a very long period of time as a typical sensitizer. Basically, a sensitizer is prepared in the form of a coating liquid containing the sensitizer by milling and dispersing the sensitizer in an aqueous medium containing basic components such as a surfactant, an antifoaming agent, a dispersing agent, etc., with a milling apparatus using a milling medium such as a ball mill, a sand mill, or the like, until it generally has a particle diameter of 5 μm or less, preferably 1.5 μm or less.
Generally, 1,2-bis(3-methylphenoxy)ethane is synthesized from 3-methylphenol and 1,2-dihalogenoethane or the like as raw materials and then purified by recrystallization. In a purified state, it is provided in the form of particles most of which have a particle diameter of 50 to 700 μm, and for incorporating it into the above coating liquid, milling thereof is essential.
That is, it is required to mill 1,2-bis(3-methylphenoxy)ethane compound (to be sometimes referred to as “phenoxyethane compound” hereinafter) in an aqueous medium containing components such as a surfactant, an antifoaming agent, a dispersing agent, etc., until it generally comes to have a particle diameter of 5 μm or less, preferably 1.5 μm or less, more preferably 1 μm or less (the particle diameter here and hereinafter refers to an average particle diameter). However, the above phenoxyethane compound is, in principle, a compound that is very hard to mill, and the step of milling the same takes a considerably long time, which is a great bottleneck in the step of producing a thermal recording medium such as a thermal recording paper, or the like, and has been a serious problem to be solved.
It is an object of the present invention to remarkably improve the milling property of the 1,2-bis(3-methylphenoxy)ethane compound excellent in basic performances such as thermal colorability as a sensitizer without impairing the above colorability, and to provide a thermal recording medium using the same as a sensitizer.
The present inventors have made diligent studies for improving the milling of 1,2-bis(3-methylphenoxy)ethane, and as a result have found that the milling property thereof is remarkably improved by preparing a specific composition having a specific content of a specific compound. Further, it has been surprisingly found that a thermal recording medium using such a 1,2-bis(3-methylphenoxy)ethane composition as a sensitizer is not degraded in colorability but is rather unexpectedly improved in colorability. The present invention has been accordingly made on the basis of the above findings.